Many applications detect an orientation of a device relative to the acceleration of gravity. One such application is an asset tag that detects the tilting of a container in which bulk product is stored to signal that the bulk product is being dispensed from the container. In this application, as in many others, the asset tag may be battery powered and is desirably as small as possible. Moreover, in this application, as in many others, for a system to be effective many asset tags may be used, and costs for a single asset tag are desirably as low as possible because those costs are multiplied by the number of asset tags that are used in an entire system.
In this asset tag application, as well as in other applications, tilt sensors are used to sense the orientation of the devices in which the tilt sensors are mounted. Traditionally, mercury switches have been adapted to serve as tilt sensors. But mercury switches are undesirable for a variety of reasons. Mercury switches pose a health hazard due to the presence of mercury. Moreover, mercury switches tend to be undesirably large and far too expensive for many applications. In applications where a need exists to sense more than one tilt angle, the large size and excessive expense problems are multiplied by the number of sensors that may be used in a single device.
An alternative to mercury switches may be found in solid sensors. Solid sensors are characterized by entrapping a solid, non-mercurous, conductive element, typically but not always spherically shaped, within a chamber. In one version of a solid sensor, the conductive element operates in conjunction with various electrical contacts that are also in the chamber. As the sensor is tilted, the acceleration of gravity causes the conductive element to move within the chamber, where it occasionally electrically shorts at least some of the contacts together. Solid sensors are highly desirably to the extent that they solve the health hazard problem posed by mercury switches. But the conventional solid sensors do not include a low power, small, inexpensive, and reliable unit.
Some solid sensors include active semiconductor components, such as optical emitters and detectors, that must remain energized in order for orientation to be monitored. Such devices consume far too much power for many low power applications. In addition, some solid sensors are configured with power-consuming circuitry, such as pull-up resistors, that in at least one orientation continuously consume a significant amount of power. These devices also consume too much power for many low power applications, and are particularly undesirable for applications where the use of more than one tilt sensor would be beneficial.
Conventional solid sensors are built using a stand-alone housing that may be mounted on a printed wiring board (PWB) but that extends above the printed wiring board more than most other components. When the sensor housing is larger than other electrical components, the sensor housing becomes a major factor in determining the size of the device, such as an asset tag, in which the sensor is used. This is an undesirable size characteristic because the sensor, more than the other components, prevents the device from being smaller. And, this size characteristic is exacerbated where the use of more than one tilt sensor would be desirable.
In addition, in battery-powered applications, tilt sensors that consume too much power cause either an undesirably large battery to be used or require the device to include special battery compartments where replaceable batteries are located. Larger batteries and special compartments for replaceable batteries lead to larger devices. And, the use of replaceable batteries, and particularly batteries that require frequent replacement, is undesirable in many applications due to the nuisance factor, the costs of replacement batteries, and the excessive unreliable operational time that must be endured when battery reserves are low.
The stability and/or reliability of conventional solid sensors has been a challenging problem. The sensor's solid conductive element should readily move under the influence of gravity so that desired tilt orientations may be detected. But this feature makes a continuous, robust electrical short between contacts difficult to make and maintain. Consequently, solid sensors tend to exhibit frequent false-open errors. False-open errors occur when the orientation of the sensor is such that a short between certain contacts should occur but does not. The false-open condition may appear only momentarily.
In fact, solid sensors can be so sensitive to movement and so unable to make and maintain a continuous robust electrical short that they are often configured as motion detectors or jitter switches rather than tilt sensors. In this configuration mere movement, even without altering tilt angle, causes the conductive element to produce a number of spurious shorts and opens between contacts. Many solid sensors are configured to heighten this effect. One way the spurious output may be heightened is to miniaturize the sensor so that the conductive element has less distance to travel within its chamber between locations where it produces contact shorts and opens. Unfortunately, while such miniaturization may be desirable for motion sensing, it tends to make solid sensors less reliable and useful when used as tilt sensors.
Some conventional solid sensors have addressed the stability and reliability problems posed for tilt sensing. But the conventional solutions have resulted in larger, more complex, more expensive components. Typically, complex structures may be included in the chamber with the conductive element to implement internal baffles, flanges, and detents with the aim of reducing spurious signals in the presence of mere movement that does not amount to tilting. In many applications where tilt sensors are needed these solutions are undesirable due to the expense and size. And, these solutions are particularly undesirable for applications where the use of more than one tilt sensor would be beneficial.